Digging with high water table

[ProDave]

12 minutes ago, Macfracam said:

The insulated slab we are looking at is extruded PS so is meant to be completely waterproof, helping if the water table rises in the winter.

I am not sure I would want an insulated EPS raft foundation below the water table.  EPS floats very well......

[G and J]

2 hours ago, ProDave said:

I am not sure I would want an insulated EPS raft foundation below the water table.  EPS floats very well......

Good for peeps named Noah perhaps….

[Iceverge]

I like the idea of trench fill foundations here. 

 

Mainly because you could dig and pour them in a few hours if you had a good ground works team and digger man.

 

Thinking out loud here and assuming the site is very wet indeed......

 

I would get a digger in and dig 4 trial/monitoring holes onsite well outside the foot print of the house. Based on the advice of your engineer I would agree a depth for the bottom of the foundations.

 

Then monitor the water level in the holes. When you were confident it was low enough, and there was zero rain forecast swiftly get a digger in, dig the trenches, consolidate the bottom, drop in some rebar and pour the trenches off a laser level. You'd only need a window of 2-3 dry days. 

 

It would need nothing near the level of accuracy of a raft or insulated raft and you'd be out of the woods regarding the major risk of the ground turning into a quagmire. 

 

I wouldn't even get rid of the topsoil beforehand as you'd be exposing yourself to getting caught out by a shower of rain that could turn your site into a swamp. That could all be done after the foundations were poured.

 

Even if the top of the foundations weren't perfectly level the imperfections could be taken out with a secondary pour or some blockwork. 

 

Someone please tell me why this wouldn't work. 

 

 

 

 

 

 

 

[ProDave]

8 hours ago, Iceverge said:

I like the idea of trench fill foundations here. 

 

Mainly because you could dig and pour them in a few hours if you had a good ground works team and digger man.

 

Thinking out loud here and assuming the site is very wet indeed......

 

I would get a digger in and dig 4 trial/monitoring holes onsite well outside the foot print of the house. Based on the advice of your engineer I would agree a depth for the bottom of the foundations.

 

Then monitor the water level in the holes. When you were confident it was low enough, and there was zero rain forecast swiftly get a digger in, dig the trenches, consolidate the bottom, drop in some rebar and pour the trenches off a laser level. You'd only need a window of 2-3 dry days. 

 

It would need nothing near the level of accuracy of a raft or insulated raft and you'd be out of the woods regarding the major risk of the ground turning into a quagmire. 

 

I wouldn't even get rid of the topsoil beforehand as you'd be exposing yourself to getting caught out by a shower of rain that could turn your site into a swamp. That could all be done after the foundations were poured.

 

Even if the top of the foundations weren't perfectly level the imperfections could be taken out with a secondary pour or some blockwork. 

 

Someone please tell me why this wouldn't work. 

 

That is almost exactly what we did except ours was not trench fill but normal foundations.

 

I dug 3 test pits for the SE to evaluate the ground, each down to 2M.  I left one open (but covered over) and monitored the water table which went anything from hole almost empty, to water almost level with the surface.

 

At the time I was building people kept asking why I was not doing an insulated and the wet ground conditions was my answer.

 

Even doing that I had an electric dirty water pump to keep the trenches dry, BC inspection one day, concrete pour next day.

[Canski]

I saw a groundwork’s crew on one site dig and pour in sections. They would have the concrete delivered into a massive skip and then pull the foundation down to the required dig level and fill with concrete using the digger bucket to move the concrete. 

[Alan Ambrose]

I'm still working on a similar problem, so the thread is very interesting. A couple of thihgs were pointed out to me recently: (1) unless you live near sea level, the actual water table / GWL is probably not just under the surface, the water we're seeing is probably some reflection of surface or perched water or maybe even water through clay using sand lenses, (2) if the water level is high then porous (sand, gravel, chalk) is actually a problem and clay is actually good - that's because the clay can be drained and won't refill quickly, but the porous stuff can't.

 

I think this is still on topic. Passing a commercial site recently which I think is for domestic housing, I saw big scrapers, dozers etc and a tanker and some tractors with 'ground consolidation' on. What's that all about?